The present invention relates generally to control systems for controlling temperatures. More specifically, the invention relates to a temperature control of steam in relation to inter-stage attemperation, which may be used in heat recovery steam generation (HRSG) systems in combined cycle power generation applications.
HRSG systems may produce steam with very high outlet temperatures. In particular, HRSG systems may include superheaters through which steam may be superheated before being used by a steam turbine. If the outlet steam from the superheaters reaches high enough temperatures, the steam turbine, as well as other equipment downstream of the HRSG, may be adversely affected. For instance, high cyclic thermal stress in the steam piping and steam turbine may eventually lead to shortened life cycles. In some cases, due to excessive temperatures, control measures may trip the gas turbine and/or steam turbine. This may result in a loss of power generation that may, in turn, impair plant revenues and operability. Inadequately controlled steam temperatures may also lead to high cyclic thermal stress in the steam piping and steam turbine, affecting their useful life. Conventional control systems have been devised to help monitor and control the temperature of outlet steam from HRSG systems. Unfortunately, these control systems often allow temperatures to overshoot during transient periods where, for instance, inlet temperatures into the superheaters increase rapidly.
Conversely, while trying to control high outlet steam temperatures, there are other potential adverse attemperation control effects. There is a danger of causing the temperature to go too low resulting in subsaturated attempertor fluid flowing through the superheaters, interconnecting piping, or steam turbine. Control stability problems can also use cyclic life of the steam system downstream of the attemperator as well as effect the life of the attemperation system valves, pumps, etc.
In particular, a non-model-based technique commonly used consists of a control structure where an outer loop creates a set point temperature for steam entering the finishing high-pressure superheater based on a difference between a desired and an actual steam temperature exiting the finishing high-pressure superheater. An outer loop proportional-integral-derivative (PID) controller may establish the set point temperature for an inner loop PID controller. The inner loop of the control logic may drive the control valve based on the difference between the actual and set point temperature to suitably reduce the steam temperature before it enters the finishing high-pressure superheater. Unfortunately, this technique may not always work to control steam temperature overshoots during transient changes in the gas turbine output. In addition, this technique may often require a great deal of tuning in order to verify satisfactory operation during all potential transients.
Regarding the overshoot problem with the non-model-based technique, as the temperature of the exhaust gas from the gas turbine increases, the temperature of the steam exiting the finishing high-pressure superheater may not only increase beyond the set point temperature, but may continue to overshoot a maximum allowable temperature even after the temperature of the exhaust gas begins to decrease. This overshoot problem may be due in part to the presence of significant thermal lag caused by the mass of metal used in the finishing high-pressure superheater. Other factors affecting attemperation may include the type and sizing of attemperation valves, operating conditions of the attemperator fluid supply pump, distances between equipment used, other limitations of equipment used, sensor location and accuracy, and so forth. This overshoot problem may also become more acute when the gas turbine exhaust temperature changes rapidly.
The conventional attemperator control logic requires an interactive and long tuning cycle. The model-based predictive technique consists of a cascading control structure where the outer loop (some combination of feedback and feed-forward) creates a set point temperature for steam entering the finishing superheater (FSH) (i.e. at the inlet of FSH) based on the difference between a desired and actual steam temperature exiting the finishing superheater (FSH). The inner loop drives the attemperator valves based on the difference between the actual and set point temperature for the inlet to the FSH to suitably reduce the steam temperature before it enters the FSH. Due to the presence of a cascade control structure the control tuning is not easy as the changes in one controller affect the performance of the other. This necessitates an interactive and long tuning cycle. Due to a competitive market and tight commissioning schedules such a controller can end up being less than optimally tuned, thus adversely affecting the long term performance of the whole system.
Accordingly, there is a need for an improved temperature control system in heat recovery systems which is easily tunable to be stable, and also prevents large temperature overshoots, and prevents the flow of subsaturated attempertor fluid through the steam system downstream of the attemperator.